In the engine the valves are in control of air intake and exhaust gases flow. With time and continuous work of the engine the valve clearances are changed due vibrations, failure of springs and securing nuts and bolts or due to wear of valve and seats. Wear of the valve gear tend to increase the clearances and as a result the power and economy of the diesel engine are degraded, the impact and wear between the parts are intensified and the noise is increased. Therefore, the valve clearance should be checked and adjusted regularly during engine lifetime and each engine model has its specific requirements. The valve clearance of the engine is generally specified by the manufacturer.
Tappet clearances are necessary, as they allow for thermal expansion of the valve spindle length at working temperature and ensure the closing and sealing of the valve against the seat during engine normal running. Clearances should normally be set while the engine is cold and the cam followers are off the cam peak (this can be checked by manually moving the pushing rods or by visual check on the camshaft which I highly recommend).
Before you proceed with the tappet clearance check and adjustment, you must make sure that you have reviewed the manufacturer’s instruction manual, where you can find engine specific procedure and data regarding the clearance allowable limits and way of measurement.
Generally, before starting the work and after reviewing the manual and taking all the safety measures (e.g. closing starting air, drain the starting air line, stop engine pre-lubrication, engage turning gear if equipped, tagging the generator “work in progress” etc.) you must ensure the following:
- engine is in the cold state – the job must be performed @30 min after engine has been stopped at the earliest as the valves should have nearly the same temperature as the cylinder head. If the clearance is measure while engine is hot, there will be “false” readings and adjustment due different materials properties and temperatures.
- open engine indicator cocks – this must be done in order to turn the engine easily as required.
- the cylinder unit of which clearance need to be checked must be on TDC – this must be done to ensure that all valves are in close position and can be verified by checking the marking on the flywheel, the position of the camshaft for that unit and/or checking the free movement of the push rods (if the push rods are tight that means that the engine is at the power stroke and should be avoided).
- check the actual clearance by using a filler gauge – this must be done in order to asses whether need to increase or decrease the clearance.
Once all the above have been fulfilled, the actual adjustment will take place and is explained in the video below from MAN Diesel:
The necessity of valve clearances check comes from normal engine valve gear wear as mentioned before or from how the engine behaves due abnormal situations such as:
- If the intake valve’s clearance is to tight there could be backfiring into the intake manifold.
- If the intake valve’s clearance is too big there will be less fresh air intake and no proper combustion will take place. This will lead to big exhaust temperature, high carbon deposits on exhaust manifold due incomplete combustion and poor engine performance.
- If the exhaust valve’s clearance is to tight, the valve will close late and this will affect cylinder compression and misfire may occur, therefore less or no power will be generated by that particular cylinder.
- If the exhaust valve’s clearance is to high the exhaust gases will not be completely removed from the cylinder and the new air-fuel mixture will be contaminated with the residual exhaust gases. This will lead to poor combustion and therefore poor cylinder and engine performance.
By checking and adjusting the clearance of these valves, the engineers responsible for these machineries are ensured that the valve’s timing is as per manufacturer’s recommendation. By following the same, we can achieve the best performance of the engine and preserving the engine and its components in good working condition over time.
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